Composing a character on a background image is called backside composition. In a case of general thermal sheets, a background image is an image of monochrome 16 gradations, and a character is black-and-white (binary). As illustrated in FIG. 1, monochrome 16-gradation image data and black-and-white binary character data are composed to obtain backside-composed 16-gradation data. FIG. 2 illustrates a specific example when monochrome 16-gradation image data and black-and-white binary character data are composed to obtain backside-composed 16-gradation data.
As illustrated in FIG. 2, image data is not covered with the white background of character data, and the character data is just composed on the image data and thereby backside composition data can be obtained.
Since a backside-composed image is generated as 16-gradation data, characters (black and white) and an image (16-gradation gray scale) can be printed at the same time.
Patent Literature 1 discloses that, in a background image printing device, a background image is represented by a light color with reduction of a stripe pattern or a checkered pattern.